Memory Loss & the Brain

An observational trial is a kind of research study in which people taking one kind of drug (or other medical treatment) are compared against people using an alternative drug or treatment. For example, a study might consider a group of 100 elderly women, 50 of whom have ever taken hormone replacement therapy (HRT), and 50 of whom have never taken HRT. If the incidence of Alzheimer's disease is lower among the 50 women taking HRT than among the 50 women who are not taking HRT, then the researchers might conclude that HRT helps prevent Alzheimer's disease.

Observational trials are most useful for suggesting a previously unsuspected link between a medication or lifestyle pattern and a disease (or prevention of a disease).

However, observational trials have several shortcomings. In clinical trials, the researcher selects which participants will receive each kind of treatment. By contrast, in observational trials, the participants themselves select their treatment, before the study even starts. This can lead to complications in interpreting the results. Thus, in the above example, it may well be the case that HRT prevents Alzheimer's disease. But it may also be the case that doctors stop prescribing HRT for women who develop Alzheimer's disease, or that women with Alzheimer's disease fail to report that they have used HRT in the past because they have forgotten. If either of these things are true, then it would appear that more women in the no-HRT group develop Alzheimer's -- even if there is no direct connection between HRT and Alzheimer's.

For these reasons, once an apparent relationship has been detected in observational trials, the next step is for researchers to conduct clinical trials. Continuing the example above, the clinical trial might consider 100 randomly-selected women, give half HRT and half an inactive placebo pill, and then see whether the women on HRT really are less likely to develop Alzheimer's disease.